Various coating compositions, methods for coating substrates with such compositions, and methods of curing the coated substrates are well known in the art. A coating composition is sometimes applied as a film, which may include a carrier (e.g., water and/or organic solvent). The coating may be applied on a substrate by a process such as lamination, solution or dispersion coating, powder coating, electrocoating, spray coaling, roll coating, or reverse roll coaling. Coating compositions can also be applied as a film by extrusion in melt form through an extrusion coating die onto a substrate. Once coated on the substrate, the composition is preferably cured (and dried if the composition includes a carrier) without defects within a few seconds, as modem high speed coating lines heat the coated substrates rapidly to temperatures of up to 490° F. (254° C.). Using such techniques, many different types of substrates have been coated including, for example, wood, plastics, and metal in the form of sheets, ships, or coils. Metal-coated substrates are especially useful in the packaging, processing, and holding of foods and beverages.
It is desirable to avoid the use of excess organic solvent in a coating method so that the environmental hazards of allowing organic solvent to evaporate into the atmosphere are minimized. However, a relatively large quantity of organic solvent is typically required in order to process the coating composition and provide a coalesced film after drying.
Epoxy resins are particularly desirable for use in protective surface coating compositions, e.g., as a vehicle or polymeric binder for optional pigments, fillers, and other additives. The epoxy resins advantageously provide properties such as toughness, flexibility, adhesion, and chemical resistance.
A number of attempts have been made in the prior art to prepare aqueous, solvent based, or solvent free coating compositions suitable for use in a process for forming cured coated substrates. For example, container coating technology has utilized an epoxy resin that has been grafted with acrylic monomers, styrene, and methacrylic acid. This grafted epoxy resin is prepared in solvent, usually butyl cellosolve and/or n-butanol, to maintain low processing viscosities, and then reduced with water by a direct or inverse let down procedure. Although the cured film properties are highly desirable, such coatings suffer from the fact that sizeable amounts of solvents are required to obtain good performance. High molecular weight epoxy resins typically require 25% to 50% solvent (based on total solids plus organic solvent) before the composition is reduced with amine and water.
Another problem that commonly arises during attempted preparation of a water based coating composition is the formation of an extremely high molecular weight product that cannot be dissolved or dispersed in water. There is a continuing need for aqueous coating compositions that can be applied to a substrate to provide excellent coating characteristics.